Thermosetting laminates

ABSTRACT

A COLD FORMABLE SANDWICH STRUCTURE WHICH HAS A THERMOSETTING RESINOUS CORE BETWEEN FACE SHEETS COMPRISING LAMINAE OF A THERMOPLASTIC SHEET WHICH INCORPORATES A METALLIC FOIL. THE METAL FOIL-THERMOPLASTIC RESIN FACING SHEETS ARE OF SUFFICIENT THICKNESS AND STRENGTH SO THAT THE SANDWICH CONTAINING THE THERMOSETTING CORE BETWEEN THE FACE SHEETS MAY BE COLD-FORMED INTO SHAPED ARTICLES AND SUCH SHAPES AS IS IMPARTED TO IT IS RETAINED BY THE THERMO-   PLASTIC FACE SHEETS WITHOUT EXTERNAL CONSTRAINT ON THE SHAPE AS THE THERMOSETTING CORE IS SUBSEQUENTLY CURED AT RELATIVELY HIGH TEMPERATURE.

Sept. 12; 1912 D. c. PREVORSEK L THERMOSETTING LAMINATES Filed June 29,1970 ZDZES E O9 SPRIN-GBACK ASSUMING LINEAR I I, I I,

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% THICKNESS PVC United States Patent Oflice 3,690,980 Patented Sept. 12,1972 3,690,980 THERMOSETTING LAMINATES Dusan C. Prevorsek, Morristown,Hsin L. Li, Parsippany, Paul J. Koch, Mount Freedom, Hendrikus J.Oswald, Morristown, and George J. Schmitt, Madison, N..l., assignors toAllied Chemical Corporation, New York,

Application Apr. 4, 1968, Ser. No. 718,858, now Patent No. 3,578,552,which is a continuation-in-part of application Ser. No. 604,255, Dec.23, 1966, now Patent No. 3,520,720. Divided and this application June29, 1970, Ser. No. 60,180 The portion of the term of the patentsubsequent to July 14, 1987, has been disclaimed Int. Cl. B3113 43/00U.S. Cl. 156-199 2 Claims ABSTRACT OF THE DISCLOSURE A cold formablesandwich structure which has a thermosetting resinous core between facesheets comprising laminae of a thermoplastic sheet which incorporates ametallic foil. The metal foil-thermoplastic resin facing sheets are ofsuflicient thickness and strength so that the sandwich containing thethermosetting core between the face sheets may be cold-formed intoshaped articles and such shape as is imparted to it is retained by thethermoplastic face sheets without external constraint on the shape asthe thermosetting core is subsequently cured at relatively hightemperature.

This is a divisional application of U.S. patent application Ser. No.718,858 filed Apr. 4, 1968, now U.S. Pat. 3,578,552, which in turn is acontinuation-in-part application of U.S. patent application Ser. No.604,255 filed on Dec. 23, 1966, now U.S. Pat. 3,520,720. This inventionrelates to the forming of shaped articles comprising syntheticthermosetting resins. More particularly, the invention relates to theprovision of a structure comprising a thermosettable plastic laminawhich is sandwiched between surface laminae consisting essentially ofmetal foil in combination with thermoplastic layers or enclosing facesheets to form a composite which may be cold-formed into suitable shapesand thereafter cured at relatively high temperatures without beingconstrained and without substantial springback.

While the relatively fast techniques which are normally applied in metalforming have been utilized, in shaping thermoplastic resin blanks orsheets these approaches when applied in connection with thermosettablecomposition have been essentially impractical. The inability to utilizethese techniques is largely due to the fact that thermosettablecompositions lack the necessary physical characteristics, e.g. generallybefore curing, they do not possess the necessary strength, drawcharacteristics, integrity to retain desired shape imparted thereto,etc. Accordingly, thermoset shapes have heretofore required considerabletime in the mold to allow setting at least to the extent of achievingpermanency of shape. This is evidenced by the normal techniques nowemployed which require confining a thermosettable plastic shape forconsiderable periods of time and a heated mold of the desired shapeuntil sufficient curing of the thermosettable composition has occurredto prevent the composition from altering the molded shape after externalconstraint on the composition has been removed.

In the aforementioned U.S. Pat. 3,520,750, formable laminates ofthermosettable resin sandwiched between thermoplastic resin layers aredisclosed. The present invention involves essentially the improvementcomprising using face sheets over the thermosettable core of athermoplastic layer in combination with a metal barrier foil which witha given thermoplastic permits the use of relatively high temperatureswithout resulting spring-back.

During a deep drawing shaping process of plastic sheet stock executed atambient temperatures, the materials undergo severe straining andstressing. Thus it is not surprising that relatively few thermoplasticpolymers are considered at present as potential candidates forprocessing by this technique. In order to perform satisfactorily in acold forming operation, in curing, and in subsequent end useapplication, the thermoplastic material must (1) have sufficientductility and strength to overcome, without rupturing, buckling,necking, etc., the stresses and strains imposed on the sheets during theforming into the desired shapes, (2) retain as much as possible theimposed shape when taken from the mold or die, and (3) retain its shapeat practical curing temperatures and (4) at temperatures of end useapplications.

Many polymers such as polystyrene, polyacrylics, etc. are not suitableunless properly modified for cold form-- ing operations because they aretoo brittle and fracture during the shaping into relatively simpleshapes. Polyethylene, polypropylene, nylon 6 and nylon 66 etc. on theother hand can be shaped relatively easy into deep shapes at ambienttemperatures; nevertheless, these materials cannot be used efficientlyin a cold forming process because of poor retention of the imposed shapeafter the removal from the die. Generally, materials that have the glasstransition temperature below the temperature of forming would usuallyspring back after release from the mold to a degree where thisdistortion cannot be economically and practically taken into account bythe appropriate design of. the mold.

Polymers such as polyvinylchloride, ABS cellulose acetate butyrate,chlorinated polyvinylchloride, etc., can be cold formed and also retainwell the imposed shape after the release from the mold. Unfortunately,items prepared from these polymers in a cold forming process undergosever distortion at moderately elevated temperatures. The heatdistortion temperature of cold formed items lies very close to the glasstransition (Tg) temperature of the polymer. Thus, it is easilyunderstood why so much eifort in cold forming has been devoted to the exploration of the potential of high glass transition polymers such aspolycarbonate, polysulfones, sulfone polyesters, etc. Most of these highTg polymers can indeed be cold formed with a relatively smallspring-back and also they retain the shape well, e.g. above thetemperature of boiling water. However, experience with these materialshas been generally unsatisfactory for a number of reasons, e.g. becauseof stress cracking, relatively high cost, greater difficulty inhandling, etc., which greatly restricts the applicability of theseotherwise very attractive materials. The problem of stress cracking isparticularly critical when these materials are used as face sheets withour thermosettable lamina. In this case the face sheets are in contactwith active components of the core during storage and shipment of theassembled laminate. Consequently, the face sheets of these laminates inaddition to the characteristics discussed above must have also: (1)resistance to stress and environmental cracking when in contact with thethermosetting core, (2) low permeability to volatiles in the core, and(3) a surface which can be bonded securely to the core during the curingprocess.

Therefore, it is not surprising that to date a satisfactory generalpurpose thermoplastic-thermoset cold formable laminate has not yet beendeveloped, i.e. the structures proposed heretofore olfer at the most asuitable compro- 1 Acrylonltrile-buta'diene s-tyrene polymers,

mies for narrow range of uses. We have found that the above-discussedproblems encountered with cold-formable thermosetting composites can besolved in an efficient, reliable and economical way by using ametal-thermoplastic combination laminate instead of a thermoplasticlayer as the face sheets. The metal foil functions as: (1)reinforcement, (2) barrier; it provides also (3) a high energy surfacefavorable for bonding; (4) reduces the spring-back on removing theshaped item from the mold; and (5 increases the heat distortiontemperature of the laminate. The latter two advantages are particularlyfavorable.

The thermoplastic layer, on the other hand, provides 1) a surface thatdoes not require before usage costly finishing and painting operations,(2) a good abrasion and chemical resistance, (3) excellent thermal andelectrical insulating characteristics, and (4) soft and warm feel, andother typical advantages characteristics of plastics.

The structures described by the present invention have a wide breadth ofutility; the sandwich assembly may be employed in fabricating a varietyof components such as vehicle fenders or bodies, housings, etc.additionally, for example metallic foil in the sandwich assembly mayserve as an electrical ground and eliminate the costly conventionalprocess of electroplating, or aluminum coating on the polymeric materialin applications where such conditions are required (e.g. in fairingsections of aircraft).

It is an object of this invention to provide a novel coldformablelaminate consisting of a cross-linkable core sandwiched between twometal-thermoplastic laminae which eliminate the disadvantage inherent insimilar laminates where the face sheets are of a thermoplastic matrial.

It is a further object of this invention to eliminate the disadvantagesinherent in cold formable metal-thermosetting laminates.

It is still a further and important object of this invention to takeadvantage of the unexpected characteristics, e.g. relatively low springback, of such metalthermoplastic-thermoset structures.

It is another object of the invention to provide a thermosettablecomposite which may readily be converted to desired shapes at ambienttemperatures characterized by a thermosettable layer sandwiched betweenthermoplastic layers in combination with metal foil laminae which are ofsufiicient strength to permit the desired shape imparted to thecomposite to be retained as the thermosettable layer is converted to thethermoset condition using relatively high curing temperatures withoutrequiring external constraint.

It is still another object of the invention to provide a compositelaminate having a relatively high heat distortion temperature comprisinga core of thermosetting composition and face sheets of thermoplasticcomposition in combination with a metal foil barrier which is capable ofbeing cold-formed, i.e. at ambient temperatures into useful shapes in anexpeditious manner.

Additional objects and advantages of the present invention will beapparent from the following description taken in conjunction with theaccompanying drawing wherein the diagram FIG. 1 depicts thedisproportionate reduction in spring back obtainable relative to thethickness ratio of metal foil in the facing lamina.

Cold formed materials have a tendency to change shape on removal fromthe die. This phenomenon which will hereafter be referred to as springback is due to the presence of stresses introduced in the item duringthe coldforming operation. Those skilled in the art will recognize thatwe are at this point concerned with the phenomenon of elastic memorywhich tends to bring back a polymeric cold shaped item into its originalform as soon as the constraining forces are removed. In cases where thismemory is very strong one can usually follow the returning of the shapeditem to its original configuration namely into a fiat sheet. Allmaterials including cold formable metals exhibit some spring back whichmust of course be properly accounted by the die-design to achieve adesired shape. It is obvious that an excessive spring back is highlyundesirable since it requires a complicated die-design and furthermoreit reduces the flexibility in the choice of shapes which can be obtainedby a cold forming process and affects the dimensional consistency of theformed articles.

Thermoplastic material, and especially those having the glass transitiontemperature below the temperature of forming, exhibit considerablespring-back from the imposed shape when the item is taken from the mold(die). One would expect that a laminate consisting of a material with ahigh spring-back and of a material with a low spring-back would exhibitan intermediate springback. We have found unexpectedly that spring-back"of the laminate approaches much closer to the springback of the materialwith low elastic memory (low spring-back) than expected on the basis ofa linear rule. For example, it is possible to use a very thin metalliclayer laminated to a thermoplastic sheet and obtain a structure withgreatly reduced spring-back.

The same unexpected effect obtained by laminating a thermoplastic and ametallic layer is observed when the heat distortion temperature of thelaminate is the consideration. We found that a very thin layer ofaluminum (e.g. 0.001") laminated to a 10 mil PVC sheet increases theheat distortion temperature of the latter from 60 C. to -l50 C. Heatdistortion temperature as the term as used herein refers to thetemperature at which a cold formed item changes one of its lineardimensions more than 5%.

Shaping and curing of the composite of the present invention may beeffected in the manner and by apparatus such as that described in thehereinabove referred to U.S. Pat. 3,520,750.

In preparing sandwich structure, it is important that the composition ofthe core and the core-outer walls thickness ratio be such that thecomposite laminate has before curing both sufficient ductility to beshaped at essentially ambient temperatures and sufficient rigidity toallow the execution of various consecutive shaping and finishingoperations without requiring, i.e. employing an external constrainingmeans such as a mold in which to retain the shape after forming anduntil curing has been effected. The outer layer of the composite,accordingly, has a multiple function: (a) to provide the desired surfaceproperties and characteristics; (b) to represent an essential structuralelement contributing to the strength and ductility required for shaping,functioning thus in a sense as faces of a matching die set and tocontinue this function during the curing step until the core isconverted into a rigid network; at this state the outer walls become anintegral part of the structure bonded securely to the rigid core. Thus,in the final state, the core reinforces the walls and etfectivelycontributes to their dimensional stability at elevated temperaturesunder load.

In forming the laminate contemplated by the invention, any of a varietyof one or more of the known thermosettable resinous compositions may beemployed to form the core layer of the composite. For example,polyesters, substituted polyesters, e.g. chlorinated polyesters,phenolics, polyurethanes, melamines, epoxies, ureas, silicones, and thelike may be used. These resins can be modified, for example, by admixingtherewith various ingredients, e.g. thermoplastics polymers such aspolyvinyl chloride, polyethylene, polystyrene, and the like. Also, theymay contain fillers, reinforcing agents, thixotropic agents and the likeand they can be prepolymerized or prethickened by some other means tothe desired viscosity. Preferably, the core compositions contain epoxy,polyester or polyurethane resins.

Epoxy compounds included in the compositions of the present inventionmay be any of the known epoxy compounds which contain a plurality ofepoxy groups of the structure Typical examples of such epoxy compoundsinclude polyglycidyl esters of polybasic acids as disclosed in US. Pat.No. 2,866,767; polyglycidyl ethers of polyhydric phenols as disclosed inUS. Pats. Nos. 2,467,171; 2,506,486; 2,640,037 and 2,841,595 andpolyglycidyl ethers of polyhydric alcohols as disclosed in US. Pats.Nos. 2,598,072 and 2,581,464.

1 Curable polyester resins contemplated for the thermosetting core ofthe invention may be any of the known compositions which contain apolymeric (a) and a monomeric component (b), i.e. one or moreethylenically unsaturated, polymerizable polyesters, polymers whichcontain, combined by ester linkages, radicals of one or more polybasic,particularly dibasic carboxylic acids and radicals of one or morepolyhydric alcohols, particularly dihydric alcohols. Optionally,additional radicals of one or more of the following may be incorporated:monobasie carboxylic acids, one or more monohydric alcohols and one ormore hydroxy carboxylic acids, at least some of said radicals havingethylenically unsaturated polymerizable groups and one or moremonomeric, ethylenically unsaturated, polymerizable compound. Typicalresin compositions of this kind are disclosed in US. Pats. Nos.2,225,313 and 2,667,430, for example. Modified resins of this kind arealso disclosed, for example, in US. Pats. Nos. 2,628,209; 3,219,604 andthe like.

Curable polyurethane resins included in the compositions of theinvention may be any known products obtained by the reaction ofpolyesters of polyethers with diisocyanates. Typical examples of suchresins are disclosed, for example, in US. Pats. Nos. 2,721,811;2,620,516; 3,061,497 and 3,105,062.

For achieving optimum properties in the final article and optimumability to shape at ambient temperature, it is important that the resincomposition of the core have a sufficiently high viscosity to prevent anexcessive squeezing out of the core material during the shaping step inthe press or mold and enough fluidity to Wet the surface of the facesheets with which it is required to obtain bonding. We have found thatfor laminates having a core to skin thickness ratio in the range from1:2 to 6:1, resin compositions having viscosities in the range from200,000 to 20,000,000 poises usually fulfills these requirements. Itshould, however, be pointed out that with thin laminates and moderatelydeep articles, the viscosity can further be reduced while with articleswith which the bonding between the outer sheets and the core is notcritical (e.g. because it is provided by some mechanical means) corescontaining higher viscosity resin compositions can be used. The corecompositions can be filled and/or reinforced, they may also containpigments, thixotropic agents, impact modifiers, etc. There are severalways to achieve and/or control the viscosity of the resin composition inthe core. For example, with polyester resins, the viscosity of the resincomposition can be increased by increasing the molecular weight of thebase resin and/or by changing the composition of the base resin or bypartial or complete substitution of styrene with some other monomerssuch as acrylamide, diallylphthalate, calcium acrylate, etc.

For economic purposes and/ or to reduce the shrinkage of the resinduring curing, it is sometimes advantageous to use fillers such as clay,asbestos, barytes, ground silica, magnesium carbonate, diatomaceousearth, etc. to increase the consistency of the resin composition. Whenhigh performance articles are the consideration, it might beadvantageous to use polyester resins modified with metal oxides,hydroxides or alkoxides (as in US. Pat. No. 3,219,604). For example, theaddition of magnesium oxide to the polyester resin results in aremarkable increase in the viscosity in comparison to a comparableunmodified polyester copolymerizable monomer composition containing thesame percentage of copolymerizable monomer. In addition, the thickeningeffect of magnesium oxide can be controlled so that the viscosityincrease takes place gradually over a period of several hours, thusassisting in the deposition of the resin applied to a glass mat in avery fluid state. In such cases a dip coating technique may be usedwhile the subsequent thickening of the resin to the desired viscositytakes place when the core is sandwiched between two thermoplasticmetallaminae. In the process good wetting of glass and face sheets isobtained which is required to obtain maximum mechanical properties andgood adhesion.

Suitable thermoplastic materials which may be employed for face sheetlaminae comprise a wide range of polymeric compositions.

Included, for example, are olefinic polymers such as polyethylene,polypropylene, and copolymers and terpolymers thereof, eg copolymers ofethylene and ethyl acrylate, vinyl polymers comprising one or more ofthe following monomers: vinyl aryls such as styrene, O-phenylstyrene,m-phenylstyrene, p-phenylstyrene, O-methylstyrene, m-methylstyrene,p-methylstyrene, o-methoxystyrene, m-methoxystyrene, p-methoxystyrene,o-nitrostyrene, m-nitrostyrene, p-nitrostyrene, and the like; vinyl andvinylidene halides, such as vinyl chloride, vinylidene chloride,vinylidene bromide, and the like; vinylesters such as vinyl acetate,vinyl propionate, vinyl butyrate, vinyl chloroacetate, vinyl benzoate,and the like; polycarbonates, that is, thermoplastics containing thefollowing repeating unit:

wherein Ar is the residue of an aromatic dihydric phenol; cellulosicssuch as cellulose acetate, cellulose triacetate, cellulose acetatebutyrate, cellulose propionate, ethyl cellulose, and the like;polyamides such as nylon 66, nylon 6, nylon 610, poly-m-xylylene,adipamide, polyhexamethylene terephthalamide, and the like; polyesterssuch as polyethylene terephthalate, polyethylene isophthalate,poly(ethylene-2,7-naphthamate), poly(ethylene-p,p'-diphenylate), variouscopolymers thereof, and

the like; chlorinated polyethylene, chlorinated polyvinyl chloride,polyfiuorocarbons such as polytetrafiuoroethylene,polytriiluorochloroethylene, polyhexafiuoropropylene and variouscopolymers and terpolymers thereof, as for example, copolymers ofvinylidene fluoride and trifiuorochloroethylene, and the like. Among thethermoplastic resins which are found to impart preferred results are:polycarbonates, polyvinyl chloride, polysulfones, cellulose aeetatebutyrate, chlorinated polyvinyl chloride and chlorinated polyethylene.

In the shaping of deep articles, particularly in deep drawn articles,below, it is advantageous to use polymers which are impact modified.This is true regardless of whether the consideration is the shaping ofthermoplasticmetal laminae per se or in combination with suitablecrosslinkable core compositions. With impact modified sheets, it ispossible to obtain both deeper draws in a single step or faster closingcycles of the press or mold, which results in a higher production rateand thus, better economy.

In addition to known chemical impact modifiers, it is also possible toachieve considerable improvement in the performance of a given polymericsheet stock in the press by treating the sheets mechanically. It hasbeen recognized by prior art that deep drawing at ambient temperaturescan be greatly facilitated if the thermoplastic sheets are either coldor hot rolled or cold or hot biaxially stretched before shaping. Inaddition to the beforementioned improvement in the draw ratio andshaping cycle, such mechanical treatment also eifects improvement in theuniformity of the thickness of the shaped article.

The foil or sheet may comprise a variety of metals such as aluminum,tin, copper, silver, gold, magnesium, steel and the like and variousalloys thereof. In general where the predominant characteristics ofplastic surface are the primary concern, metal foils of from less than 1mil may be used. Thicknesses of metal foil to as much as 100 mils ormore may be employed. The metal to plastic thickness ratio of the facesheets may vary from about 0.01 to about 10.

The following examples illustrate the invention in greater detail. Partsrecited are parts by weight unless expressly stated otherwise.

EXAMPLE I This example illustrates the preparation of articles shaped atroom temperature from a sandwich consisting of a reinforced epoxy resininterposed between two laminated face shets. The laminated face sheetswere prepared by bonding aluminum foil to polyvinyl chloride. Thefollowing composition is suitable for shaping by the below-describedtechnique:

Polyvinyl chloride 1 (each of two sheets 0.010" thick) Aluminum foil(each of two sheets 0.003 thick) Fiber glass reinforced epoxy resin corecompound of the following composition:

Parts/wgt. General purpose bisphenol A type epoxy resin Aluminumpalmitate (Qutco Chemical Co.) 1.2

The above-described core compound was prepared in a sigma blade mixeruntil a homogenous putty was obtained. The polyvinyl chloride-aluminumfoil laminates were prepared by adhesive bonding under heat andpressure. The sandwich assembly, whose total thickness was 0.110", wasconstructed so that the aluminum foil faced the core. A 7 /2" diametercircular blank was cut from this assembly and shaped, using a hydraulicpress and triple action, deep drawing die set, into a diameter flatb0ttomed cup whose wall height was 1.5." The die set works as follows:first, the blank-holder applies a present pressure to the flange of theblank, then the punch draws the blank from under the blank-holder intothe die to shape the cup. Finally, the knock-out pushes the cup from thedie as the punch and blank-holder return to their starting position.

The shaped assembly was removed from the press, placed in an aircirculating oven and allowed to cure at 93 C. for two hours. The curedcup was then exposed an additional hour at 150 C. with no observabledistortion.

An all polyvinyl chloride cup, and a similar sandwich assembly using allpolyvinyl face sheets had a heat distortion temperature of 70 C.

Commercially available as 1145-0 from Aluminum Corporatlon of America.

8 EXAMPLE II The procedure of Example I was generally repeated using theepoxy core composition described. However, instead of polyvinylchloride-aluminum laminates as face sheets, there separate runs usingface sheets (a), (b) and (c), described below were substituted.

Run (a) used laminated face sheets of 0.010" thick polyethylene and0.003" thick aluminum foil, run (b) used 0.010" thick polypropylene and0.003" thick aluminum foil and run (c) used 0.010" thick nylon 6 and0.003" thick aluminum foil. The laminated sheets, as described, wereassembled into a composite sandwich with the epoxy putty core. Articlesshaped from these composites by the previously described technique, werecured at 93 C. for two hours and further exposed an additional hour at150 C. with no observable distortion.

EXAMPLE III This example illustrates the preparation of shaped articlesfrom a sandwich assembly consisting of a layer of reinforced polyesterresin interposed between two laminated face sheets. The face sheets wereprepared by bonding aluminum foil to polycarbonate. The followingcomposition is suitable for shaping by the previously describedtechnique.

Polycarbonate 1 (two sheets of 0.020" thick each) Aluminum foil (twosheets of 0.003" thick each) A layer of non-woven glass fiber clothimpregnated with a polyester resin composition having a viscosity ofabout 800,000 centipoises (amount of glass -25% by weight) 2 comprisesthe core.

The sandwich assembly was constructed so that the aluminum foil facedthe core. A 7 /2" diameter circular blank was cut from this assembly andshaped into a 5" diameter fiat bottomed cup whose wall height was 1.5".

The shaped assembly was removed from the press, placed in an aircirculating oven and allowed to cure at C. for 1 hour, then C. for anadditional hour. There was no evidence of stress cracking in thepolycarbonate portion of the face sheets.

When a similar sandwich assembly, using all polycarbonate face sheets,was shaped and cured, the polycarbonate became severly stress cracked.

EXAMPLE IV The procedure of Example III was generally repeated using thepolyester core composition described. However, instead ofpolycarbonate-aluminum laminates as face sheets, polysulfone 3 (0.020"thick)-aluminum foil (0.003" thick) laminates were substituted. Aftershaping and curing there was no evidence of stress cracking in thepolysulfone portion of the face sheets.

When a similar sandwich assembly, using all polysulfone face sheets, wasshaped and cured, the polysulfone became severly stress cracked.

EXAMPLE V This example illustrates the importance of using thecombination of polyvinyl chloride and metal foil in the face sheetslaminate. The structure is assembled into a composite sandwich with anepoxy putty core. This sandwich can be shaped into an article at roomtemperature and cured at elevated temperature outside the shapingapparatus. The disadvantage of using polyvinyl chloride alone oraluminum foil alone as face sheets is illustrated as follows:

(a) A cup was formed, using the composition and technique as describedin Example I, with the reception 1 Commercially available as Lexanpolycarbonate from General Electric Company.

Commercially available Allied Chemical Corporation polygster Eesin soldunder the trademark Plaskon 750 can also a use Commercially available asPolysultone 3500 from Union Carbide Corn.

that only polyvinyl chloride and no aluminurfi foil was used in the facesheet. This cup was placed in an air circulation oven and allowed tocure at 150 C. for /2 hour. The polyvinyl chloride face sheetscompletely delaminated from the core.

(b) The experiment was repeated as illustrated above except thataluminum foil 1 (each of two sheets 0.003 thick) was used as face sheetsin place of polyvinyl chloride. Excessive wrinkling and buckling of thealuminum foil along the radial direction and severe rupture alongcircuferential direction of the cup were observed.

In a third run face sheets of aluminum foil (0.003" thick) bonded topolyvinyl chloride sheet (0.010") under heat and pressure were employed.Excellent cups were formed using the same procedure describedpreviously. The cup was placed in an oven at 150 C. for /2 hour; nodelamination was observed.

EXAMPLE VI This example illustrates the unexpected finding thatspringback, that is the percent dimensional change from the imposeddimension, does not follow a linear rule between high and low springbackmaterials, but that the springback of a laminate approaches much closerto that of the material with the low springback.

Face sheet laminates of polyvinyl chloride and aluminum foil wereprepared over a range of polyvinyl chloride thicknesses (.005" to .020")and aluminum thicknesses (.001" to .010"). Composite cups were producedwith these laminates as face sheets, by techniques described in ExampleI, and their springback, expressed as percent increase in the innerdiameter of the cup from that imposed by the punch, were measured.

On the basis of a linear relationship, one could expect, for example,that a laminate of .010" polyvinyl chloride and .003" aluminum wouldhave a springback of 5%, however, a much lower value of 0.6% wasactually observed. Additional values of measured springback andcalculated values assuming a linear relationship between springback ofthe laminate and its composition are given in the figure of the drawingand Table I. The overriding influence of the low springback materialextends well into the region where the thickness of themetal-thermoplastic laminate consists of 90% or more of thethermoplastic component.

It will be appreciated by those skilled in the art that analogousbehavior will result where heat distortion expressed as percent changein dimension at elevated temperature is the consideration.

1 Commercially available as 1145-0 from Aluminum Corporation of America.

TABLE I Polyvinyl springback chloride, Aluminum, Polyvinyl thickness inthickness chloride, Calculated, Observed, inches in inches percentpercent; percent We claim:

1. A method for forming articles from sheet plastic material comprisingin combination the steps of: disposing a lamina of thermosetting resinbetween layers consisting essentially of a metal foil and thermoplasticresin to form a layered composite assembly, shaping said compositeassembly to the desired shape in a forming apparatus at ambienttemperature, said composite being retained in the forming apparatus fora period of time suflicient to shape the thermoplastic layers into thedesired shape and said thermoplastic layers having at the time offorming the composite suflicient rigidity to retain the shape impartedto said assembly, removing said shape from the forming apparatus withthe thermosetting lamina in a substantially uncured state, and curingthe thermosetting lamina of said composite while said shape is free ofexternal constraint.

2. The method of claim 1 wherein said curing is efiected at temperaturesin excess of the glass transition temperature of the thermoplastic resinlayer.

References Cited UNITED STATES PATENTS 3,132,416 5/1964 Hait 156-443 X2,797,179 6/1957 Reynold et a1. 264-257 2,796,634 6/ 1957 Chellis 156196X 3,457,130 7/1969 Morrison 156432 X 2,962,764 12/1960 Trojanowski etal. 2643 16 X 3,492,392 1/1970 Kasamatsu et a1. 264-316 X 3,436,2974/1969 Brooks et a1 156273 X 3,082,485 3/1963 Thomas 156-496 X 3,393,1197/1968 Dugan 156-273 X BENJAMIN A. BORCI-IELT, Primary Examiner H. J.TUDOR, Assistant Examiner U.S. Cl. X.R. 156-459, 585

